The invention relates to an acoustic device, and in particular to an acoustic device of the type that uses resonant bending wave modes.
Prior resonant bending wave devices are described in WO97/09842 and U.S. counterpart application Ser. No. 08/707,012, filed Sep. 3, 1996 (now U.S. Pat. No. 6,332,029) (the latter application being incorporated herein by reference in its entirety). These documents describe a panel having resonant bending wave modes in the area of the panel. A transducer may be provided at a preferential location on the panel for exciting the resonant modes. Such a device is known as a distributed mode loudspeaker. Operated in reverse, the device is a distributed mode microphone.
U.S. Pat. No. 3,347,335 describes a loudspeaker in which bending waves are sent along a beam. In this device the bending waves are excited at one end of the beam and a nonreflecting termination is provided at the other end. Since the termination is non-reflecting, the bending waves will travel down the beam, be absorbed and will not reflect back to form resonant modes.
According to the invention there is provided an acoustic device comprising a member having a modal axis along which axis there are a plurality of resonant bending wave modes, and non-modal axes perpendicular to the modal axis, wherein the fundamental frequency of the resonant modes along each non-modal axis is at least five times the fundamental frequency of the resonant modes along the modal axis.
Preferably, the fundamental frequency of the resonant modes along each non-modal axis is at least ten times the fundamental frequency along the modal axis. The higher the fundamental frequency along the non-modal axis compared to along the modal axis, the more the acoustic device can be said to be xe2x80x9cone-dimensionalxe2x80x9d.
The member may be a panel with the modal axis along the length of the panel and a non-modal axis along the width of the panel. The panel need not be flat.
When a resonant bending wave mode is excited in a panel it will cause the panel to displace by a small amount out of the plane of the panel. The amount of this displacement will vary along a direction in the plane of the panel, and it is the direction along which the displacement varies and not the direction of the displacement itself that is meant when a bending wave mode is said to be along a particular direction.
The fundamental frequency along a particular axis is the frequency of the lowest bending wave mode along that axis. The density of modes along an axis is related to the fundamental frequency along that axis: in a broad frequency range there will be more resonant modes along an axis with a low fundamental frequency than along an axis with a higher fundamental frequency.
For comparison, the prior art documents WO97/09842 and U.S. Ser. No. 08/707,012 teach interleaving the frequencies of the modes along the long and short axes, which requires similar fundamental frequencies. That document teaches isotropic panels with aspect ratios of 1.134 or 1.41, which correspond to ratios of fundamental frequencies of 1.285 and 2 respectively.
The fundamental frequency fo along an axis of a panel may be related to the panel bending stiffness B (about a perpendicular axis) and the panel length L along the axis by the proportional relationship (which assumes constant mass per unit area)
(fo)2xcex1B/L4.
It will be seen that in order to achieve a high ratio of the fundamental frequency along the width axis over that along the length axis the width may be less than half, preferably less than a third of the length.
The sound emitted from a panel is anisotropic at frequencies where resonant bending wave modes along the modal axis, but not the non-modal axis, are excited. In such frequency ranges sound is preferentially emitted into a plane perpendicular to the panel through the modal axis, and reduced in a plane perpendicular to the modal axis through the non-modal axis. This can give rise to enhancement of the sound into the plane through the modal axis at these frequencies. Accordingly the panel may be particularly suitable for use with piezoelectric transducers, which have a frequency response which tails off at low frequencies. The increased low frequency sound output can compensate for this tailing off of excitation to provide a more even sound overall.
The preferential sound radiation into a single plane can also be useful in some specific applications, for example to direct sound into a horizontal plane in a room and avoid sending too much sound to a ceiling or floor of the room.
The preferential emission of sound into a plane is greatest for a flat panel, rather than a rod, and increases with increased width. However, this assumes that the one-dimensionality can be maintained and that modes along the non-modal axis of the panel are not excited. This latter condition requires a narrow width. In order to achieve the contradictory requirements of one-dimensional behaviour but with a panel of significant width a highly anisotropic panel may be used.
The panel may be stiffer to bend about the modal axis than about the non-modal axis. The bending stiffness of the panel about the modal axis panel may be at least 1.5 times that about the non-modal axis, further preferably at least twice as stiff. Since the resonant bending wave modes along an axis cause bending about a perpendicular axis, if the panel is stiffer to bend about the modal axis this will reduce the number of modes along the non-modal axis.
A panel having anisotropic bending stiffness may be made of a material having a corrugated or cellular structure, with the cells or corrugations running in the plane of the panel along the non-modal axis.
In embodiments, a transducer may be provided to excite the resonant bending wave modes. The transducer may preferably be placed at a location which is spaced away from the nodes of the lower modes along the modal direction. To achieve this, the transducer may be placed at a preferred location along the length of the member, for example at substantially 4/9, 3/7 or 5/13 of the length along the modal axis. These locations are similar to those taught in WO97/09842 and U.S. Ser. No. 08/707,012, except that in those documents the preferred locations have these coordinate values in both directions. The transducer need not be placed on the modal axis, but may be placed laterally thereof.
A plurality of transducers may be provided. To provide multiple transducers at one preferred location a plurality of transducers may be placed side by side across the width of the panel. This can provide increased output. Alternatively, a single transducer may extend across the width of the panel at a preferred location. Such a transducer can be effective even if it only causes bending along one axis.
A bending transducer extending across the width of the panel may be able to provide greater power than a single point-like transducer for use on a two-dimensional panel which cannot have a significant spatial extent.
It may also be possible to excite the panel at a less-preferred location, for example a location nearer one end than the preferred location. It is possible to vary the bending stiffness along the modal axis so that positions other than those mentioned above become preferred. Alternatively, it may be possible to damp or clamp the panel in some way to improve the efficiency of the panel even when excited at a less preferred location.